The present invention relates to a soft magnetic powder composite core, especially a high frequency soft magnetic powder composite core for use in high frequency transformers, reactors, thyristor valves, noise filters, choke coils and the like, a process for forming insulating layers on the soft magnetic particles suitable for the core, a treatment solution for forming the insulating layers, and an electric device with the soft magnetic powder composite core.
The cores for high frequency coils which are used for high frequency transformers, reactors, thyristor valves, noise filters, choke coils and the like should not only have a low iron loss and a high magnetic flux density, but also have magnetic properties which do not get worse even in high frequency regions.
The iron loss includes an eddy current loss which has a close relation with a resistivity of core, and a hysteresis loss which is greatly influenced by strains in iron particles caused in the process of production of the iron particles and post-processing history thereof.
The eddy current loss increases in direct proportion to the square frequency, so it is important to lower the eddy current loss in order to improve the properties at high frequencies. Lowering the eddy current loss requires molding of soft magnetic particles under compression into a core and to have the soft magnetic powder composite cores structured with each soft magnetic particle being insulated so that eddy currents are confined in small domains.
However, if the insulation is not sufficient, the eddy current loss becomes large. It may be considered to thicken the insulating layers to improve the insulating property. However, a thicker insulating layer results in a lower magnetic flux density due to a reduction in the proportion of soft magnetic particles in a core. Alternatively, an attempt to increase the magnetic flux density by compression-molding under high pressures may lead to larger strains in the shape, hence to a higher hysteresis loss resulting in an increase in iron loss.
In order to manufacture a soft magnetic powder composite core having better characteristics, therefore, it is important that the resistivity of the core is increased without reducing the density. For this reason, it is necessary to cover iron particles with a thin insulating layer having a high insulating property.
The soft magnetic powder composite cores have heretofore been produced by processes where the insulating layers are made of organic binders such as fluorinated resins or inorganic binders such as polysiloxanes and water glass as disclosed in Japanese Patent KOKAI (Laid-open) Nos. Sho 59-50138, 61-154014 and 51-89198. In order to obtain sufficient insulating properties by these processes, however, it is necessary to increase the thickness of the insulating layers which results in reduced magnetic permeability.
An attempt has been proposed to solve the above problems by subjecting soft magnetic particles to a coupling treatment and then mixing with binder resin, followed by molding under pressure as disclosed in Japanese Patent Publication No. Hei 6-11008. However, in this process the resistivity cannot be sufficiently increased though the higher density may be achieved.
In order to overcome the difficulties as above, there has been proposed a process for forming thin insulating layers on magnetic particles without lowering the density where the layers having excellent properties can be formed by treatment of a phosphate salts solution. This phosphating treatment is disclosed in Japanese Patent KOKAI (Laid-open) Nos. Hei 6-260319, Sho 62-22410, and Sho 63-70504.
It has been found, however, that even using any of these processes, it is difficult to sufficiently increase the resistivity of the magnetic core without lowering the density.
In the prior art, there has been no treatment solution for forming insulating layers which allows formation of thin layers having good insulating properties on iron particles, nor a process for producing soft magnetic particles which have thin and highly insulating layers coated on the surfaces and a high formability under compression. Therefore, it has been difficult heretofore to produce a soft magnetic powder composite core having a sufficiently low iron loss and a sufficiently high magnetic permeability.
An investigation has been made to find out the causes of the insufficient resistivity and magnetic permeability of prior art soft magnetic powder composite cores which were made with soft magnetic particles having insulating layers formed by using conventional insulating layer-forming phosphate solutions. As a result, the following have been found:
When iron particles are treated to form insulating layers thereon, rust is produced on the iron particles. The rust may cause a reduction in formability under compression which leads to an insufficiently high magnetic flux density. Depending upon the heat-treatment conditions, there may be produced a sort of iron oxide (rust), i.e., electro-conductive Fe.sub.3 O.sub.4 which causes a reduction in electric resistance as well as an increase in eddy current loss of a magnetic core which is produced by pressing the particles.
Taking account of the foregoing, it has been found that the generation of rust at the time of treating the soft magnetic particles for forming insulating layers thereon must be prevented in order to obtain a soft magnetic powder composite core having excellent characteristics.
On the other hand, Japanese Patent KOKAI No. Hei 1-220407 discloses a soft magnetic powder composite core which was produced by treating soft magnetic particles with a rust inhibitor such as benzotriazole and then mixing them with a binder resin and molding the mixture under pressure into a magnetic core. This method effects suppression of this generation of rust by oxygen or water present in the air after the production of the soft magnetic powder composite core. However, this method cannot solve the aforementioned problems that the resistivity of soft magnetic particles is raised and the iron loss is reduced.
If a phosphating treatment is performed after the rust inhibiting treatment to expect realization of both rust inhibition and insulating effects, the formation of insulating coatings does not proceed uniformly, resulting in a reduced resistance which causes a high eddy current loss, though the generation of rust may be suppressed.
Since the solutions for the phosphating treatment are an acidic aqueous solution containing a high concentration of ions and the treatment is performed at high temperatures, a corrosion current is generated at the time of formation of the insulating layers so that the generation of rust occurs on the surfaces of iron particles to render the formation of insulating layers uneven.
From the foregoing, it has been concluded that there is a need for a solution for phosphating treatment which has an intense electronic interaction with the surfaces of iron particles and an effect of preventing the generation of rust due to the suppression of the generation of corrosion current and which does not adversely affect the formation of insulating layers. The present invention has been achieved based on this conclusion.